During the continuous casting of steel, molten metal is transferred from large ladles to intermediate pouring vessels called tundishes. From these tundishes, the metal is poured into ingots, or other molds, to cast the molten metal.
While in the tundishes, the molten steel will not always have sufficient residence time to allow non-metallic inclusions to become separated from the liquid metal. This generally occurs when the metal is quickly transferred from the tundishes to the molds from which the continuously cast steel is formed.
Impurities entrained in the metal may affect the final cast product. Processing of the metal may be thereby hampered, or the finished product may be less pure, less structurally sound or may have poor finishing quality due to its anodizing characteristics. These impurities may originate from several sources. For example, impurities may include metallic impurities such as alkaline and/or alkaline earth metals, dissolved entrained gas and occluded tundish surface oxide films which have broken up in the molten metal. In addition, inclusions may originate as insoluble impurities such as carbides or borides and eroded furnace and trough refractories.
When these non-metallic inclusions do become separated, their lower density allows them to rise to the top of the tundish in the form of slag. This slag is then skimmed from the molten metal. It is therefore desirable to remove these inclusions while residing in the tundishes.
It is therefore desirable to increase the residence time of the metal in the tundish. This allows the metal to settle enough for entrained gases and non-metallic inclusions to be removed in the form of slag.
It is also desirable to form the tundish in a shape that maintains a relatively homogeneous temperature throughout the interior surface of the tundish. In other words, it is undesirable to have molten metal occupying "dead zones" in the tundish where the molten metal does not circulate. Dead zones result in a less homogeneous metal, and also reduce the effective capacity of the tundish, since some of the metal will remain in the tundish for an excessively long time.
In order to improve the circulation of the molten metal and also increase residence time, tundishes have been developed in which the molten metal flow is diverted using barriers. These barriers are usually, but not necessarily, substantially rectangular in shape. They extend between the tundish walls in a direction which is transverse to the metal flow between the metal entry and exit points.
One type of barrier, commonly called a weir, is located in the upper part of the tundish. The weir prevents liquid metal from flowing continuously across the surface of the molten metal, while allowing liquid flow beneath the weir. Another type of barrier, commonly called a dam, is located in the lower part of the tundish, typically protruding from the tundish floor. In a tundish equipped with a dam, the flow of liquid metal is directed to move over the top of the dam. Dams and weirs may be used together in order to produce a desired liquid metal flow pattern.
A preferred barrier are baffles which is configured similarly to the weirs or dams previously described. The baffles compartmentalize a tundish into an inlet and outlet side and contain, below the normal liquid level of the steel present in the tundish, a plurality of holes. The molten steel flowing through the holes positioned in the baffle, which in certain instances are angled to provide a good flow pattern, substantially increase the uniformity of temperature of the steel and also provide a good flow pattern which tends to diminish dead zones.